1. Field of the Invention
The present invention relates to the field of bandgap references.
2. Prior Art
Bandgap references are well known in the prior art, and are commonly used in integrated circuits to provide a reference that is independent of temperature. These references make use of two characteristics of the base-emitter voltage (VBE) of a bipolar transistor. In particular, the base-emitter voltage VBE of a junction transistor may be expressed as follows:       V    BE    =            V      g0        +                  (                              V            BE0                    -                      V            g0                          )            ⁢              (                  T                      T            0                          )              +                  NKT        q            ⁢              ln        ⁡                  (                                    T              0                        T                    )                      +                  KT        q            ⁢              ln        ⁡                  (                                    I              C                                      I              C0                                )                    
where:
T=temperature
IC=the transistor collector current
IC0=collector current for which VBEO was determined
Vg0=bandgap voltage of silicon at temperature T0 
VBE0=base to emitter voltage V at T0 and ICO 
q=electron charge
N=structure factor
K=Boltzmann""s constant
The dominant terms are the first two terms:       V    g0    +            (                        V          BE0                -                  V          g0                    )        ⁢          (              T                  T          0                    )      
and since Vg0 is larger than VBE0, the net result is a negative temperature coefficient for the VBE of a transistor.
If one subtracts the VBEs of two identical transistors Q1 and Q2 operating with unequal collector currents, there results:                               V          BE1                -                  V          BE2                    =                                    KT            q                    ⁢                      ln            ⁡                          (                                                I                  C1                                                  I                  C0                                            )                                      -                              KT            q                    ⁢                      ln            ⁡                          (                                                I                  C2                                                  I                  C0                                            )                                            ⁢          xe2x80x83                  or      :              
            ⁢                        V          BE1                -                  V          BE2                      =                  KT        q            ⁢              ln        ⁡                  (                                    I              C1                                      I              C2                                )                    
This frequently is expressed in terms of current densities J1 and J2 in the two transistors as follows:             V      BE1        -          V      BE2        =            KT      q        ⁢          ln      ⁡              (                              J            1                                J            2                          )            
or for transistors that are of different areas (area ratio of 1 to n) but otherwise identical and having the same collector currents, can be expressed in terms of the transistor areas A as follows:             V      BE1        -          V      BE2        =                    KT        q            ⁢              ln        ⁡                  (                                    A              2                                      A              1                                )                      =                  KT        q            ⁢              ln        ⁡                  (          n          )                    
In bandgap references, two transistors are usually operated at different current densities, typically by using two transistors of different areas, but having equal collector currents. Accordingly, for specificity in the descriptions to follow, it will be assumed that the respective two transistors have different areas and have substantially equal collector currents, though this is not a specific limitation of the invention, as transistors of the same area could be operated at different collector currents, or transistors of different areas could be operated at different collector currents in the practice of the present invention.
Now referring to FIG. 1, a circuit diagram for a classic bandgap reference may be seen. In such a circuit, resistors R2 and R3 could be equal resistors with amplifier A1, preferably a high input impedance amplifier, driving the output voltage VBG to the voltage required to provide a zero differential input to the amplifier. Accordingly, under these conditions, the currents through resistors R2 and R3 are equal currents, and accordingly, neglecting the base currents of transistors Q1 and Q2, provide equal collector currents to transistors Q1 and Q2. In such a circuit, transistor Q2 could have an area n times the area of transistor Q1, so that the current density in transistor Q2 is only 1/n times the current density in transistor Q1.
Amplifier A1 forces the collector voltages of transistors Q1 and Q2 to be equal. Because the collector voltages are equal, the voltage VR1 across resistor R1 is as follows:
VR1=VBEq1xe2x88x92VBEq2
Where:
VBEq1 is the base emitter voltage of transistor Q1, and
VBEq2 is the base emitter voltage of transistor Q2
Referring back to the prior equations, it may be seen that the difference in these two VBE""S, the voltage across resistor R1, is proportional to absolute temperature. Also, since the current in resistor R2 equals the current in resistor R1, the voltage across resistor R2 is also proportional to absolute temperature, and can be thought of as amplifying the voltage across resistor R1 by a factor of (R1+R2)/R1.
In addition to the voltages proportional to absolute temperature (PTAT) across resistors R1 and R2, that leg of the circuit also includes the base emitter voltage VBE of transistor Q2. Again, referring to the prior equations, the VBE of a transistor linearly decreases with increases in temperature. Accordingly, by proper selection of the value of resistor R2 in relation to the value of resistor R1, the linear rate of increase in the PTAT voltage across the combination of resistors R1 and R2 with temperature increase may be made to equal the linear rate of decrease of the base emitter voltage VBE of transistor Q2 with temperature increases, so that the bandgap voltage output of the circuit VBG is substantially temperature insensitive.
In typical prior art bandgap references, the area ratio for transistors Q1 and Q2 may be, by way of example, on the order of 10 to 1, which area ratio will provide a VBE difference, the voltage across resistor R1, on the order of 60 millivolts. The output voltage of the bandgap reference needed to balance the positive temperature coefficient of the voltage across resistors R1 and R2 with the negative temperature coefficient of the VBE of transistor Q2 for a silicon transistor is typically a little over 1.2 volts. Accordingly, resistor R2 typically is approximately an order of magnitude larger in resistance than resistor R1.
The resistor R2 effectively amplifies the voltage across resistor R1, including the noise across resistor R1. In a typical bandgap reference circuit, resistor R1 is the single largest source of wideband noise. The noise across resistor R1 includes not only the thermal noise of resistor R1, but also the shot noise of transistors Q1 and Q2, and for that matter, the noise associated with the base resistance of transistors Q1 and Q2.
In electronic systems, the voltage reference provides the known standard that the rest of the system relies upon. Electronic circuit noise present in voltage references can limit the overall accuracy and ultimately the usefulness of the reference. Previous methods of reducing noise have depended on increased circuit power consumption or expensive semiconductor process development. The present invention improves the noise performance of bandgap references using a new circuit arrangement with existing process technology.
Low noise bandgap references of the type providing a temperature independent output by balancing the proportional to absolute temperature dependence of the difference in base-emitter voltages of two transistors operating at different current densities with the negative temperature coefficient of the base-emitter voltage of a transistor are disclosed. The bandgap references disclosed reduce the noise characteristic of such references by balancing the difference in base-emitter voltages of a first number of pairs of transistors, each pair having two transistors operating at different current densities, with the negative temperature coefficient of the base-emitter voltage of a second number of transistors, the second number being less than the first number. Various embodiments are disclosed, including embodiments having an output corresponding to the bandgap of the transistor material (silicon in the exemplary embodiment), and multiples of the bandgap of the transistor material.